Imaging devices, as thus broadly characterized, have been achieved in the prior art. For example, FIG. 8 shows a construction of a prior art charge coupled device (CCD) imaging element. The imaging element of FIG. 8 is based on an n-type semiconductor substrate 28, a p-type epitaxial layer 27 disposed on substrate 28, and a photodetector 23 comprising an n-type layer disposed in and at the surface of layer 27. The photodetector 23 serves to collect electrons generated in response to incident light. A CCD transfer section 26 comprises an n.sup.- -type layer disposed in layer 27 for receiving stored electrons from photodetector 23. A first CCD transfer gate 24 forms an embedded channel in layer 27 to read out the quantity of electrons stored at photodetector 23, and a second CCD transfer gate 25 disposed on first transfer gate 24 transfers electrons at CCD transfer section 26. In an array of these imaging elements, each element is separated from adjacent elements by an oxide film 22.
When light from an object is incident on photodetector 23, electron-hole pairs are generated. The electrons among the pairs are collected by photodetector 23, and the quantity of electrons collected corresponds to the intensity of the incident light. When a voltage is applied to control gate (first transfer gate) 24, an inversion layer (a non-equilibrium n-type layer) is produced in p-type epitaxial layer 27 opposite gate 24. The inversion layer produces a channel through which electrons may flow between the n-type layer of photodetector 23 and the n-type layer of CCD transfer section 26. The electrons collected by photodetector 23 flow into CCD transfer section 26 comprising an n.sup.- -type layer. Electrons that flow into CCD transfer section 26 are driven in accordance with the potential well at CCD transfer section 26. That well is produced by clock signals applied to first transfer gate 24 and to second transfer gate 25 so that the electron quantity can be read out as a signal. In an array of CCD elements lying along two transverse directions, by sequentially and repeatedly operating the CCD elements as described, it is possible to convert the light signal from an image into an electrical signal.
In the prior art system imaging device, incident light is converted to an electrical signal by a pn junction photodiode. Electrons generated by this conversion are stored for a predetermined time and thereafter are transferred to CCD transfer section 26 under the control of gate 24. Electrons that flow into CCD transfer section 26 are further transferred in response to the modulation of the potential well by clock signals applied to transfer gates 24 and 25. Thus, an electric signal corresponding to the image of a material object is obtained.
In the prior art imaging device, a photodiode for the light-to-electricity conversion and the transfer channel elements, that is, spaced apart diffused impurity regions and aligned gates, have to be constructed for each picture element on a single semiconductor substrate. As a result, the aperture ratio of the picture element (the proportion of the area occupied by the photodiode relative to the entire area of the picture element) is limited. That limitation not only limits resolution, but is also an obstacle to large scale integration of the imaging device.
Furthermore, it is impossible to improve the sensitivity of the element to incident light beyond a given threshold because the light-to-electricity conversion uses a pn junction photodiode. Moreover, the clock speed is limited because of the limited speed at which it is possible to store the light-generated electrons.